My point being is that someone should compile a database of children who have been shot by a sniper during a conflict. Use that database to get a list of descriptions of known snipers who committed such acts and start to narrow that list. Maybe some where killed during the conflict, some are in jail, some are leading quiet lives in Oshkos, etc...When that list is narrowed down, I would check with private security firms who specialize in protecting businessmen overseas who must have a list of top notch assassins to narrow it down even further. Look to link that list with any of them who has done work for Muslims before LIKE THE BOSNIAN MUSLIMS and you may have something to go on.
Well, I don't know who committed the most atrocities -- didn't keep score. I do know all sides in the conflict committed plenty. (Serbs, Croats, "Bosnian Muslims" and "Kosovar Alabanians.") I've never been in Chechnya, but from what I understand, that conflict has been most grisly with atrocities committed on both sides and both sides using snipers.
If you're looking for a flipped-out former merc, well, all above-mentioned ethnic groups from the former Yugoslavia have gone on to be mercs in African countries (sometimes associated with the FFL, sometimes not) where children as young as, sometimes younger than, the boy shot on Monday are often armed combatants. There were also some German mercs working for the Croats (don't know they were snipers) and a few Dutch mercs on several sides. Also the occasional Russian "volunteer" on the Serb side. And we all know about mercs or "jihadders" from various Moslem countries operating in Bosnia. Some are still there.
It's also possible the shooter didn't consider a 13-year-old as a "child." After all, he's well, er, not like the rest of us.